Wireless Local Area Network (WLAN) devices, such as those conforming to Institute for Electrical and Electronic Engineers (IEEE) 802.11 protocols, may utilize a power management technique that coordinates delivery of data through the use of a traffic indication message (TIM). When a STA is in power save mode and is unable to actively receive data, data may be buffered for later transmission at the AP. The AP indicates the availability of buffered data pending transmission by sending a TIM at regularly scheduled intervals. A STA in power save mode awakens to receive the TIM and upon determining that buffered data is pending, sends a power save poll (PS-Poll) frame to the AP. Receipt of a PS-Poll frame does not change the power management status associated with the STA and the AP continues to classify the STA as being in power save mode. Thus, the AP transmits the first pending data frame in response to the PS-Poll frame. Rather than waiting for the next TIM interval, the AP may signal the STA when additional data is pending by setting the More Data bit of the packet header to signal its availability. Correspondingly, when the STA receives a data frame with the More Data bit set, it may send subsequent PS-Poll frames to individually trigger the transfer of additional frames until it receives a data frame in which the More Data bit is not set, thus informing the station that all currently buffered data has been delivered.
Under this implementation, an undesirable overhead may be imparted to the system due to the requirement that the STA send separate PS-Poll frames to initiate the transfer of each data frame, consuming bandwidth and transmission opportunities for other nodes in the network. To address this issue, the STA may instead signal a change in power management status. Rather than a PS-Poll frame, the STA may unset the Power Management bit in the header of a frame sent to the AP, indicating that it is exiting power save mode. Upon receipt of this frame, the access point then associates an active status with the STA and may deliver all pending data frames without prompting from the station.
However, under conventional 802.11 protocols, there is no provision for the AP to use the More Data bit when communicating with a STA that is in active mode under normal circumstances. As a result, a STA in active mode has no means for determining whether there is data pending transmission at the AP and a delay in returning to power save mode may be imparted. For example, the station may not return to power save mode until an inactivity timeout period has lapsed. The timeout period is typically set to a sufficiently long duration to accommodate any delays in transmission of pending data from the AP, such as when the network is congested and the AP has to contend for access to the medium. Accordingly, the STA may remain in active mode for the inactivity timeout period even though there may be no additional buffered traffic at the AP, representing an undesirable consumption of power.
In addition to the legacy power save mode operation described above, use of the More Data bit to signal completion of buffered traffic delivery has been suggested in other power save contexts. For example, U.S. Pat. No. 6,917,598 describes a legacy power save mechanism triggered by a STA having uplink voice traffic. It is predicated on the assumption that the AP will have a corresponding downlink frame, given that the STA and AP are configured with the same voice frame duration. This patent also describes an improvement in the form of an unscheduled power save delivery (UPSD) mode, that relies on negotiating recurring service periods to ensure a baseline quality of service (QoS) for a reserved access category, such as voice traffic. In both mechanisms, usage of the More Data bit is associated with communications of a certain priority and is limited to defined time periods, either in the form of the voice frame duration or the recurring service period. As such, these techniques are not applicable to the delivery of generic data that may be buffered at the AP.
Accordingly, this disclosure includes systems and methods for providing communication regarding the existence of data for a STA that is buffered at an AP whenever the STA is in active mode. Correspondingly, the STA may efficiently enter power save mode upon determining the buffered data has been delivered.